Israeli researchers report in PNAS this week that embryonic pig tissues used for liver, pancreas, and lung transplants need to come from very specific windows of time in embryonic development.

The findings offer new insights into organogenesis and may help explain past failures in xenotransplantation, coauthor Yair Reisner of the Weizmann Institute of Science in Rehovot, Israel, told The Scientist.

Reisner explained that although research into using embryonic pig tissues as a source of transplantable organs has gone on for more than two decades, timing of the transplant is a challenge. Too early, and undifferentiated embryonic tissue may develop into a teratoma, he said. Transplant too late, and embryonic tissues may have been marked with identifiers that trigger rejection by the new host.

"Studying these windows provides a great assay system for basic research regarding the timing of developmental events at the molecular level," Reisner said.

In 2003, Reisner and colleagues defined the gestational windows for the maximum capacity of human and pig kidneys to grow and differentiate into functional tissues, with minimal risk for teratoma formation. In the most recent study, they transplanted pig tissue precursors from embryonic day (E)21 to E100 into immune-deficient mice to identify the optimal windows for the growth of liver, pancreas, and lung.

The researchers saw optimal liver growth and function at E28, with enzyme-linked immunosorbent assay (ELISA) showing rapid decrease of secreted albumin levels from implants past that stage. In contrast, development of mature lung tissue was not observed via stained micrographs until relatively late in gestation at E56. Maximal pancreas growth and function was seen from E42 to E56, with ELISA revealing reduced insulin secretion capacity before and afterward.

"Disappointing results in past transplantation trials may be explained, at least in part, by these results," Reisner said. "Early studies that attempted to cure diabetic patients by implantation of pig embryonic pancreas made use of late gestation tissue, which is now shown to be inferior compared to the optimal 6 weeks gestational time."

The authors also found that teratomas were common after liver implants younger than E28, but they saw no potential for teratoma in pancreas or lung tissue at any time point. This could be explained by different relative amounts of pluripotent and committed stem cells in each developing organ or perhaps the differentially expressed restricting activity of stromal elements.

"We all look for genes which characterize pluripotent stem cells," Reisner said. "Considering that the ability to form teratoma reflects the presence of pluripotentiality, we have now defined the loss of such stem cells within E24 and E28 in the liver. Clearly, this will allow us now to probe genes which disappear within these narrow gestational time points."

Marc Hammerman at Washington University in St. Louis, who did not participate in this study, noted that information gathered from studies in immunodeficient mice might not correlate with what goes on in immunocompetent hosts. "But I think these observations are still very valuable in better understanding organogenesis," he told The Scientist.

"This study has identified critical checkpoints in [organ] development, and it has important implications to help identify what transcription factors get activated here and there, and what factors are needed to move from one stage to the next," said Bernhard Hering of the University of Minnesota in Minneapolis, who did not participate in this study.